The proposed research is divided into six parts, each of which involves the application of dynamic NMR spectroscopy to problems in conformational analysis. The projects of the first three parts are expected to provide experimental evidence that "aromaticity" is important for stabilizing the Z conformations of esters, secondary amides (proteins), and related compounds. Vinyl, ethynyl, phenyl and cyclopropyl groups are expected to be incapable of providing the third pair of electrons to complete the aromatic sextet, and compounds containing these groups will be studied to determine whether the populations of E-isomers are large, as expected. The ability to control the conformation of aromatic amides is known to be useful in the design of drugs, and the proposed research is expected to provide a set of groups which can be used to change the conformation of certain esters, amides, and related compounds, in addition to contributing to an understanding of the conformational preferences of three important classes of biological compounds (amides, esters, and thioesters). In part I, low-temperature 17O, 14N, and 13C NMR spectroscopy will be used to study E-Z isomerism in thionitries and a selenonitrite. Part II describes a study by 13C and 1H NMR of conformational equilibration in several thioesters of acetic acid. In part III, acetylenic esters of a carboxylic acid and a thiol acid will be studied by 1H and 13C NMR. Parts IV, V and VI describes studies of trifluorothioacetic acid, several cyclohexyl compounds that can have both E-Z and axial-equatorial isomers, and several lactones, respectively.